1. Field of the Invention
The present invention relates to a method and apparatus for controlling the length of a carbon nanotube. In particular, the present invention relates to a method and apparatus suitable for modifying the tip of a carbon nanotube probe, a carbon nanotube sensor or a carbon nanotube field emitter.
2. Description of Related Art
Carbon nanotubes are used widely and an example of such use is as probe tips for scanning probe microscopes (SPMs), measurement of the mass of nano particulars or sensors for detection of whether atoms or molecules exist. A probe microscopic cantilever can be made by the current micro-electromechanical technology. An array of probes having carbon nanotube tips can be formed on a p-type wafer, an n-type wafer, a glass substrate or the like substrate by chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) or field-enhanced chemical vapor deposition (FECVD). This is exemplified by U.S. Pat. No. 6,146,227, which provides an advantageous solution for mass-production of SPM probes by disclosing a process for growing a carbon nanotube on a probe tip. In addition, U.S. Pat. No. 6,346,189 issued to Dai et al. in 2002 disclosed an atomic force microscopic probe to have a catalyst disposed on its probe tip on which a carbon nanotube is grown by CVD.
Although recent research has focused on the growth of the carbon nanotubes, a number of problems remain in reference to obtaining carbon nanotubes of uniform quality. Taking the length of the carbon nanotubes as an example, a control of time for growing the carbon nanotubes does not necessarily mean that the length of the tip extending from the carbon nanotubes can be adequately controlled. This is because the positional control of the catalyst particles is difficult, for example, the catalyst particles may not be formed on the end of the probe tip. Thus, carbon nanotubes grown by CVD are of different lengths extending from the probe. On the other hand, the carbon nanotubes grown adhere to the surface of the probe by van der Waal's force, and then, crawl along the slant surface of the probe tip. When the internal force of the carbon nanotubes is larger than the adherent force between the carbon nanotubes and the surface of the probe, the carbon nanotubes extend from the probe tips. Therefore, the length of the probe tips formed under the same condition and the same procedure will vary. The carbon nanotubes of non-uniform length cause a number of problems in practice. For example, a lengthy SPM tip tends to be effected by thermal disturbance and results in vibration, though it is instrumental to the probation of a deeper microstructure. Also, the lengthy carbon nanotube tip tends to cause buckling. If the length of the tip is used inappropriately, the outcome of scan will vary. As a result, the accuracy of size measurement of a sample object is lessened, reducing the effect of the carbon nanotube probes. Moreover, as application is made to measure the mass of the nano particles, small particles adhere to the end of the carbon nanotubes having a rigidly mounted end and a free end. Since the length of the carbon nanotubes has an effect on the vibration condition of the carbon nanotubes, the mass of the nano particles is measured based on the specific resonance frequency of a carbon nanotube structure corresponding to the specific length of the carbon nanotubes. As for the application to serve as a sensor for detection of whether atoms or molecules exist, a carbon nanotube having organic functions is used to catch chemical materials. Hence, it is necessary to adequately control the length of the carbon nanotube sensor tip, to make the sensor to function well. Furthermore, the carbon nanotubes having the catalyst particles at the end probably effects field emission of electrons. Hence, the researchers for the field emission of electrons also care about innovations on methods for cutting the catalyst particles at the end of the carbon nanotubes.
In recent years, several efforts working on the length-controlling of carbon nanotubes are reported. For example, cutting a carbon nanotube through the assistance of a voltage pulse between the tip of the scanning tunneling microscope (STM) and a single nanotube is described by Liesbeth et al. (Liesbeth et al., Applied Physics Letters, Vol.71, No.18, 1997). Another method for controlling the length of a single carbon nanotube is also disclosed by Hafner et al. (Hafner et al., The Journal of Physical Chemistry B, Vol. 105, No. 4, 2001). The single carbon nanotube is picked up and adhered to a tip of single-walled carbon nanotubes through the assistance of UV-cure adhesive first in Hafner's method. Then a voltage pulse is applied to the tip for cutting the carbon nanotube. However, all these methods illustrated above need to be achieved through a complex process or expensive apparatus.
It is necessary for the conventional techniques to control the length of the carbon nanotube tips by going through the processing steps for every probe. Both the complex processing steps and expensive apparatus prevent high-quality carbon nanotubes from popularization in the industry. Absent supply of the carbon nanotube probes retards the development of researches and applications of nano-scale measurement. Furthermore, even though several complex methods for cutting a single carbon nanotube are disclosed, no simple method or cheap apparatus for mass-producing the carbon nanotubes with uniform length is disclosed.